To protect oil and gas pipelines, and also the ballast tanks of ships, from corrosion, coatings are to be provided that withstand cathodic protection.
Unless stainless steel or certain marine bronzes are used to manufacture such metal articles, corrosion, with its associated aesthetic problems and failure modes can be expected to severely limit product lifetime. Even when these two relatively corrosion-resistant classes of materials are used, corrosion may still take place, particularly in salt water or brackish environment. Corrosion problems are most severe when more active metals such as magnesium, aluminum, and carbon steel are used. Such items may become severely corroded over relatively short periods of time.
To lessen the corrosive effects on metals, it has been common to provide surface treatments. Chrome and nickel plating have been used, for example. However, plating is relatively expensive, particularly when large fabricated structures constructed by welding are to be plated. In addition, such plating procedures do not work well on many active metals such as aluminum.
Anodizing has also been used to increase corrosion resistance, and is effectively used on small parts. However, large tubular structures are typically welded together. The anodized coating is destroyed locally during the welding process. Anodizing very large, prefabricated structures is not cost-effective.
Several coating systems to provide corrosion resistance have been proposed in the prior art.
Performance properties such as sandability, recoatability and corrosion resistance are particularly important for coating compositions intended for use as primers over steel substrates. However, it has been difficult for the prior art to obtain the proper balance with regard to sandability, recoatability, corrosion resistance, and metal adhesion requirements.
Failure to provide adequate corrosion resistance or salt spray resistance typically manifests as “scribe creep”. “Scripe creep” refers to the degree of corrosion and/or loss of adhesion which occurs along and underneath film adjacent to a scribe made in a cured film after the scribed film has been placed in a salt spray test apparatus. The scribe generally extends down through the film to the underlying metal substrate. As used herein, both corrosion resistance and salt spray resistance refer to the ability of a cured film to stop the progression of corrosion and/or loss of adhesion along a scribe line placed in a salt spray test apparatus for a specified time. Cured films that fail to provide adequate salt spray resistance are vulnerable to large scale film damage and/or loss of adhesion as a result of small or initially minor chips, cuts and scratches to the film and subsequent exposure to outdoor weathering elements.
Coal tar enamels, asphalt, and epoxy coal tar paints have previously been used as anti-corrosive coating compositions. These coating compositions have a number of drawbacks, inter alia, they are poor in low temperature characteristics such as curability, brittleness, impact resistance and flexibility. For example, epoxy coal tar paints, while having good adhesion properties, have poor coating efficiency and abrasion resistance because cure time is extensive thereby hindering the application of relatively thick coatings.
Fusion bonded epoxide (FBE) systems, applied as a primer (optionally 2 coats) are known for coating pipelines. Here the powder is reacted on the hot pipe. The FBE coating is coated with polyethylene/polypropylene (PE/PP) by the sintering process.
A disadvantage of the prior art is that the FBE coatings require substrate temperatures of 180 to 240° C. in order to cure and bond to the metal substrate. This constitutes a high energy cost.
Also FBE cured coatings can withstand operating temperatures of no more than 140° C., which is not sufficient for deep well oil exploration pipelines; the FBE anti-corrosion coating will be damaged at temperatures around 140° C.
Polyurethane coatings for metallic substrates are known, see US 2003/0139561, U.S. Pat. Nos. 5,391,686, 4,716,210, WO 02/051949, U.S. Pat. No. 6,699,528, WO 96/33816 and WO 01/79369.
Two-component mixtures, a polyol component and a polyisocyanate component, are generally reacted at an isocyanate index of between 80 and 150% to form the coating.
U.S. Pat. No. 6,387,447 and WO 2010/003788 describe syntactic thermal insulating coatings for pipes employed in the offshore sector. These coatings are obtained by reacting a polyisocyanate with an isocyanate-reactive compound in the presence of hollow objects.
Although polyurethane coatings have been known to be useful as primers, they have not achieved the desired balance of properties.
In particular, for polyurethane films to provide desirable corrosion resistance, they have typically relied upon the use of corrosion protection components containing heavy metal pigments such as strontium chromate, lead silica chromate, and the like. Unfortunately, sanding such a film produces dust that is environmentally disfavored due to the presence of the heavy metal containing pigments. Accordingly, it would be advantageous to provide a coating which can provide adequate corrosion resistance but which is substantially free of any heavy metal containing pigments.
Further the rigid polyurethane foams which are customarily used today are designed for continuous operating temperatures of up to 130° C. with short peaks of up to 140° C. This is adequate for most Western European district heating networks. Eastern European power stations, however, supply substantially higher flow temperatures, which may reach 200° C. The rigid polyurethane foam which is customarily used is not suitable for such temperature ranges.